The cathode ray tube (CRT) is the most common form of video display device and is used widely for television sets, computer terminals, and various other video display purposes. The CRT is an evacuated chamber in which a hot cathode emits electrons which are directed with focusing elements toward a transparent, phosphor-coated screen. As the electrons strike the screen, they are absorbed by the phosphorescent coating on the screen which emits visible light seen by a viewer looking at the other side of the screen.
Typically, the phosphor screen of a CRT is divided into a number of small spots, called picture elements or "pels", each of which may be separately illuminated by the cathode electrons. By directing electrons from the cathode to illuminate only desired pels, an image may be formed on the screen which is made up of that selected group of pels. Since the phosphorescent screen material glows for a finite period of time after absorbing the electrons from the electron beam, sequentially illuminating the desired pels fast enough results in an image displayed on the screen which appears to be simultaneous and continuous. The time which a phosphor glows after absorbing a particular amount of electron beam energy is called the "persistence" of the phosphor.
Traditionally, the source of electrons for a CRT is an "electron gun" which uses a cathode to emit electrons which are formed into a single, linear electron beam. The electron gun then accelerates and focuses the electron beam using a series of controllable field-generating elements. These elements generate electric or magnetic fields which control the intensity and the direction of the electron beam. Usually, the electron beam is "scanned" across the screen one row at a time. The scan must be therefore be fast enough to "refresh" the glowing phosphor of the screen to prevent perceived image discontinuity.
One of the problems encountered in the design of CRT's is how to achieve a high density of pels on the CRT screen. As the electron beam is focused by the electron gun focusing elements, and as it travels toward the screen, the mutually repulsive electrostatic forces of the electrons comprising the beam force the beam to spread apart. For a particular beam current, voltage, and shape, a minimum spot diameter exists beyond which the beam can not be focused. This, in turn, limits the density of pels on the screen.
The focus spot diameter may be reduced by reducing the beam current, which consequently reduces the repulsive electrostatic forces. However, reduction of the beam current also reduces the electron beam energy absorbed by each pel of the screen phosphor. Traditional methods for increasing the number of pels in an image beyond some nominal limit include: 1) increasing the beam voltage, which reduces beam spreading and increases pel brightness, 2) use lower beam currents and oblige viewers to sit in a darkened room, 3) use more highly converging beam shapes with wider diameters in the focusing region, and 4) use multiple beams simultaneously.